The following four spectral line backends are available which can be individually connected to any single pixel receiver and, if indicated, also to HERA.
The 1 MHz filterbank consists of 4 units. Each unit has 256 channels with 1 MHz spacing and can be connected to different or the same receivers giving bandwidths between 256 MHz and 1024 MHz. The maximum bandwidth is available for only one receiver, naturally one having a 1 GHz wide IF bandwidth. Connection of the filterbank in the 1 GHz mode presently excludes the use of any other backend with the same receiver.
Other configurations of the 1 MHz filterbank include a setup in 2 units of 512 MHz connected to two different receivers, or 4 units of 256 MHz width connected to up to four (not necessarily) different receivers. Each unit can be shifted in steps of 32 MHz relative to the center frequency of the connected receiver.
The 100 KHz filterbank consists of 256 channels of 100 KHz spacing. It can be split into two halves, each movable over a range of 500 MHz, and connectable to two different receivers.
VESPA, the VErsatile Spectrometric and Polarimetric Array correlator, can be connected either to HERA or to a subset of 4 single pixel receivers, or to a pair of single pixel receivers for polarimetry. The many VESPA configurations and user modes are summarized in a Newsletter contribution [14] and in a user guide, but are best visualised on a demonstration program which can be downloaded from our web page at URL ./IRAMFR/PV/veleta.html. Connected to a set of 4 single pixel receivers VESPA typically provides up to 12000 spectral channels (on average 3000 per receiver). Up to 18000 channels are possible in special configurations. Nominal spectral resolutions range from 3.3 KHz to 1.25 MHz. Nominal bandwidths are in the range 10 -- 512 MHz. When VESPA is connected to HERA, up to 18000 spectral channels can be used with the following typical combinations of nominal resolution (KHz) and maximum bandwidth (MHz): 20/40, 40/80, 80/160, 320/320, 1250/640.
The 4 MHz filterbank consists of nine units. Each unit has 256 channels (spacing of 4 MHz, spectral resolution at 3 dB is 6.2 MHz) and thus covers a total bandwidth of 1 GHz. The 9 units are designed for connection to HERA, but a subset of 4 units can also be connected to the backend distribution box which feeds the single pixel spectral line receivers. All these receivers have a 1 GHz RF bandwidth except for A100 and B100 (500 MHz only). At the present time, a 4 MHz filterbank cannot be used simultaneously with the autocorrelator or the 100 KHz filterbank on the same receiver.
An on-line calibration routine automatically writes calibrated spectrometer data, including the 4 MHz filterbanks, to the Linux machines. The routine, although still experimental, works for all observing modes. A logical link named ``data.30m'' pointing to this file of calibrated spectra is made available in all newly created project accounts.
The new autocorrelator WILMA consists of 18 units which connect to the 18 detectors of HERA. Each unit nominally provides 512 spectral channels, spaced out by 2 MHz and thus covering a total bandwidth of nominally 1 GHz of which 930 MHz are usable. Each band is sliced into two 500 MHz subbands which are digitized with 2 bit/1 GHz samplers. An informative technical overview of the architecture is available on the backend section (URL: ./IRAMFR/TA/backend/veleta/wilma/index.html) of our web pages.